Improving fMRI sensitivity by normalization of basal physiologic state

The power of fMRI in assessing neural activities is hampered by inter-subject variations in basal physiologic parameters, which may not be related to neural activation but has a modulatory effect on fMRI signals. Therefore, normalization of fMRI signals with these parameters is useful in reducing variations and improving sensitivity of this important technique. Recently, we have shown that basal venous oxygenation is a significant modulator of fMRI signals and individuals with higher venous oxygenation tend to have lower fMRI signals. In this study, we aim to test the utility of venous oxygenation normalization in distinguishing subject groups. A "model" condition was used in which two visual stimuli with different flashing frequencies were used to stimulate two subject groups, respectively, thereby simulating the situation of control and patient groups. It was found that visualevoked BOLD signal is significantly correlated with baseline venous T2 (P 1/4 0.0003) and inclusion of physiologic modulator in the regression analysis can substantially reduce P values of group-level statistical tests. When applied to voxel-wise analysis, the normalization process can allow the detection of more significant voxels. The utility of other basal parameters, including blood pressure, heart rate, arterial oxygenation, and end-tidal CO2, in BOLD normalization was also assessed and it was found that the improvement was less significant. Time-to-peak of the BOLD responses was also studied and it was found that subjects with higher basal venous oxygenation tend to slower BOLD responses.